There is a known electromagnetic module, MTT of Sondex, to identify defects in tubing strings (Magnetic thickness tools-MTT. Leading Oilfield Technology. Sondex, p. 10, http://www.sondex.com). The MTT consists of one exciter coil and 12 miniature magnetic sensors, mounted on the inside of bow springs. The generator operates at three frequencies. The signal amplitude and phase at the pick-up coil depend on the amount of metal surrounding a sensor.
The drawback of this known logging tool is that, while it can identify defects in one tubing string or an interval of the casing string after withdrawal from tubing, it cannot distinguish between defects on the inner and outer sides of a string. Moreover, this logging tool cannot see hole-type defects less than 15 mm in diameter.
There is a known device that can identify defects and perforations in pipe strings. The downhole electromagnetic defectoscope includes a housing, an exciter coil with its magnetic axis being aligned with the tool's axis, and a pick-up coil with its magnetic axis being perpendicular to the tool's axis (RF patent No. 2215143, published 27 Oct. 2003, E21B49/00, G01N27/90). In this known device, an alternating current is fed into the exciter coil to generate loop eddy currents in the surrounding steel pipe, and those, in turn, induce an EMF in pick-up coils. As the pick-up coils pass by defects in the pipe string, characteristic changes in the magnetic field are observed.
The drawback of this device is its inability to distinguish between defects on the inner and outer surfaces of a pipe. Moreover, a defect can be detected if it is in close proximity to the pick-up coil.
There is a method and a device to inspect objects, including pipes, using moving sensors, known from the U.S. Pat. No. 5,233,297 (Atlantic Richfield Company, published 3 Aug. 1993, IPC G01N 27/72, G01N 27/82, G01R 33/12, G01B 7/10). The device contains an active region with a transmitting element and at least one pick-up element (sensor). The transmitting element is connected to a pulse current source that generates a magnetic field and is, in turn, connected to a microcontroller. The active region is in close proximity to the walls of a surveyed object, so that the sensors touch its walls and are equidistant from one another. The active region moves along the object and the transmitting element induces a pulsed current in the object's walls. The sensors detect the currents induced in the object's walls after every current pulse and transmit the received signals for processing. Then, these data are interpreted to determine the thickness of the object. The object's wall thickness is determined by comparing the received signal with the reference one that is generated using an object of known thickness. The drawback of the device is that it measures only local pipe wall thicknesses.
There is a device for downhole casing control known from the Author's Certificate No. 972895, published on 23 Mar. 1992, G01 N 27/87. The device contains numerous sensors located on the circumference of a casing string in one plane. The sensors consist of cores and magnetic paths separated by a non-magnetic spacer or air clearance, and of coils with outputs. The magnetic flux through defects is higher than that through undamaged areas, which is the main criterion for defect detection.
The drawback of this device is that it can be used to detect only large defects. Moreover, its design requires a large size of the device. Therefore, it can only be used in large-diameter pipes, for instance, in gas pipelines.
There is an electromagnetic method, known from the U.S. Pat. No. 7,960,969, Schlumberger Technology Corp., published on 14 Jun. 2011, IPC E21B 47/08, for the electromagnetic measurement of the physical properties of pipes using a device containing multiple transmitting and pick-up coils connected so that they form multiple measuring devices designed to be placed and moved in a pipe. Physical properties are measured at multiple positions in the pipe. The essence of this known method is as follows. A magnetic field is generated in the object under study by current pulses through an exciter coil. The response of this field is recorded in the tool's memory as the time dependence of the EMF of output signals from uncompensated sensors placed equidistantly from one another on the cross-section circumference of the casing pipe under study. Then, the recorded signals are processed to identify defects on the cross-section circumference of the casing pipe under study.
This solution is the closest in its technical essence to the proposed group of inventions. The main drawback of this known solution is that the magnetic field response is measured by a measuring device that cannot measure the response signals of the secondary electromagnetic field if most response signals of the primary electromagnetic field are simultaneously compensated, because it contains, among others, uncompensated sensors only. This narrows the dynamic range of the magnetic field's response signal for small defects, which makes their detection impossible and decreases the resolution.
Thus, state-of-the-art analysis shows that the known technical solutions cannot ensure detection of small defects, such as 3-5-mm holes, in casing strings. Moreover, small defects, such as holes larger than 15 mm in diameter, can be detected if they are in close proximity to the sensor.